Thermal-magnetic trip units used within residential and commercial molded case circuit breakers are generally limited by geometric considerations from providing low current magnetic trip response. U.S. Pat. No. 4,513,268 describes a residential type molded case circuit breaker incorporating a thermal-magnetic trip unit in accordance with the prior art. U.S. Pat. No. 4,951,015 describes a movable core that is designed to move into the gap existing between the core and armature of a magnetic trip unit to reduce the primary air gap and increase the magnetic flux. The movable core effectively allows the circuit breaker to trip at lower current levels. U.S. Pat. Nos. 3,179,767, 3,278,707 and 3,278,708 each describe the use of an additional turn of wire around the magnet used within the thermal-magnetic trip unit to increase the magnetic forces on the armature at low currents.
Additionally, U.S. patent application entitled "Thermal-Magnetic Trip Unit" Ser. No. 07-841180 describes a pivotally-arranged intermediate armature assembled between the latching armature and fixed magnet used within residential circuit breaker thermal-magnetic trip units. The additional intermediate armature correspondingly decreases the magnetic separation gap between the magnet and latching armature to increase the magnetic trip response.
One purpose of the invention is to provide a circuit breaker low cost thermal-magnetic trip unit having improved low current trip response without requiring an additional armature or any substantial changes to the circuit breaker trip unit.